1. Field of the Invention
The present invention relates to a process and apparatus for distillation separation of stable isotope atoms present in the form of a stable isotope compound using a distillation column packed with formed packing. More specifically, the present invention relates to a process and apparatus for distillation separation using a distillation column packed with a so called "promoting-fluid-dispersion type" structured packing. By employing this process and apparatus, optimal separation of .sup.13 CO and .sup.12 CO can be obtained.
This application is based on patent application No. Hei 9-279223 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Among stable isotope atoms, .sup.13 C, for example, is naturally existing in C at a ratio of 1.11%, and is naturally existing in CO at the same ratio in the form of .sup.13 CO.
A variety of isotope separation methods are available, including separation by thermal diffusion, gaseous diffusion, centrifuge, laser, chemical exchange, and distillation. In the case of CO isotope separation, a distillation separation method has conventionally been employed.
In the case of .sup.13 C separation in the form of .sup.13 CO, starting material CO is typically separated into .sup.12 CO and .sup.13 CO using one or more distillation columns. Either, .sup.13 CO is enriched in .sup.12 CO from material CO by distillation.
The separation of an isotope gas mixture by distillation is characterized that a separation coefficient is extremely close to 1. In the case of a .sup.2 CO/.sup.13 CO distillation, for example, the relative volatility, that is separation coefficient, is 1.005-1.009. For this reason, in order to obtain .sup.13 CO having a purity of 99.9% or higher, 2000 or more theoretical plates are required. Moreover, a reflux flow rate as much as 1000-fold or more of draw-off rate at the bottom is needed. Accordingly, .sup.13 CO and .sup.12 CO separation has typically been carried out using a plurality of distillation columns.
FIG. 9 shows an example of a conventional .sup.13 CO separation and manufacturing apparatus. This conventional apparatus employs a plurality of distillation columns to produce 99.9% pure .sup.13 CO. In the figure, the symbols 1A, 1B and 1C indicate distillation columns, 2A, 2B and 2C are reboilers, and 3A, 3B and 3C are heaters.
In this apparatus, distillation column 1A has a column diameter of 25 mm, and is packed with a random packing such as Heri-pak (1.3.times.2.5.times.23 mm wire forms, manufactured by Pedbelniak, Inc.). Distillation column 1B has a column diameter of 50 mm and is packed with a random packing such as Pro-pak (manufactured by Scientific Development Co.). In general, when scaling up distillation columns employing random packing, it is necessary to increase the column diameter in response to the flow rate in the column to be processed. However, the selection of a random packing appropriate for the column diameter can be difficult, because of a reduction in the efficiency of distillation occurring due to muldistribution of the liquid. Thus, in order to maintain distillation efficiency, therefore the increase of the packed height is needed.
However, there is of course a limit to the height of the distillation column. Thus, in the case that the production rate exceeds the flow rate which can be processed by one column, a method has been employed wherein the number of distillation columns are increased in proportion to the production rate. This approach is inconvenient, however, as the composition of the apparatus becomes complicated.
In the above-described conventional system, when separating .sup.13 CO and .sup.12 CO to produce .sup.13 CO, if the column diameter is increased from 25 mm to 50 mm, then the flow rate which can be processed increases by 4-fold. Further, by changing the packing, an 8-fold increase in the processing flow rate is possible, however, the column height increases by 2.5-fold. Consequently, in order to separate .sup.13 CO and 12CO using a 3-stage distillation column system such as shown in FIG. 9, to produce .sup.13 CO having a purity of 99.9% at a rate of 2 mol/day, a design is necessary in which there are six distillation columns 1A having a column diameter of 50 mm and a column height of 100 m, and one distillation column 1B having a column diameter of 50 mm and a column height of 100 m.
Furthermore, the liquid hold-up employing this type of random packing is 20-30% of the column's internal volume, or 40-60% occasionally the case of a large amount. Thus, approximately 150 days are required from the time the apparatus begins operation until the entire column reaches the condition of steady state operation. This factor accordingly adds a considerable burden in terms of cost and the production schedule. This conventional method is reported in detail in the following reference:
B. B. McInteer, Los Alamos Scientific Laboratory, "Isotope Separation by Distillation Design of a Carbon-13 Plant", Separation Science and Technology, 15(3), pp 491-508 (1980).
Conventional distillation process use random packing, employing apparatus in which a number of enormous columns are provided standing side by side. Thus, the cost for constructing the apparatus is considerable as compared to the increase in production rate. Moreover, expanding the column capacity may be planned, however, no benefit of scale is obtained.
The demand for .sup.13 C has grown in recent years accompanying its expanding applications. Accordingly, it has become necessary to increase production of this product. As one solution for increasing production, the column diameter may simply be increased as described above. However, this approach is difficult to carry out due to the requirement of increasing the column height. Moreover, if the number of existing apparatus is merely increased, no benefit from scale is obtained.
Therefore, when increasing the diameter of the column, it has become necessary to seek benefits from scale by specifying the state of gas-liquid contact, i.e., by specifying a packing having a shape and structure which satisfies fixed conditions, the packing method, and the operating conditions at which a maximum production rate can be obtained from a distillation column packed with that packing.
A method is also known for increasing production in which a new apparatus having a different structure is connected to the existing equipment in the form of a cascade. Namely, in this method, .sup.13 CO that has been concentrated in the new apparatus is fed into the existing equipment, then concentrated and separated. By means of this method, production can be increased without changing the volume processed by the existing apparatus. In addition, equipment installation costs can be held in check.
In this case, however, consideration must be given as to how to incorporate the new apparatus's processes to match the volume processed by the existing apparatus. Moreover, in order to keep equipment costs low, it is preferable that the new apparatus have a small column diameter and column height.